This invention relates generally to x-ray image processing techniques, and more particularly to methods and apparatus to correct distortion in pixellated x-ray imaging detectors with energy resolution.
A pixellated x-ray detector with energy resolution can be made using semiconductor materials that directly convert x-rays into electron-hole pairs. Two effects distort x-ray spectra that can be measured by a pixilated x-ray detector. One is pileup of x-ray events resulting from the inherent statistical nature of the counting process. A second is sharing of charge between adjacent pixels due to the finite size of the charge cloud generated by the x-ray beam. The size of this charge cloud is dependent upon the diffusion of charge carriers outward from the x-ray interaction site, and also depends on the time allowed for diffusion to take place. This time, in turn, is dependent upon the distance from the interaction site to the collecting electrode. In this way, charge sharing is dependent upon detector thickness.
Some applications such as mammography require high spatial resolution and thus, small pixel sizes. However, the size of the charge cloud can be comparable to the size of the required small pixels. Simulations of a 100 micron pixel and 0.5 mm thickness detector show that 70% of x-ray events are subject to some degree of charge sharing.
The nature of the x-ray spectrum distortion resulting from charge sharing can be quantified. Depending upon the manner in which pileup is handled by the electronics of an x-ray imager, pileup can either cause events to be counted together that should instead be counted separately (thereby resulting in higher energy events occurring in a collected spectrum) or cause a loss of a second event that occurs within the dead time of the detector. Charge sharing creates the appearance of additional low energy events because the energy of a single x-ray gets divided between more than one pixel, effectively adding counts to the low end of the energy spectrum and removing counts from the high end.